This invention relates to bearings, and, more particularly, to a method for producing a filament wound thrust bearing assembly having an inner race and an outer race integrally formed thereon and the bearing made thereby.
Thrust bearing assemblies comprise an inner member, or race having an axis and an outer race with the inner facing bearing surfaces of the inner and outer races having at least one axial thrust resisting means, or area. To permit relative rotational movement between the inner and outer race the adjacent bearing surfaces and thrust resisting area must be annular. When axial thrust is to be resisted bi-directionally, thrust resisting areas must be oriented to face both axial directions.
Bi-directional thrust resisting areas can be provided by forming the outer race over an inner race that has an outer surface having peripheral radial irregularities. The radial irregularities take the form of one or more annular troughs or grooves, or annular elevations or beads. The inner surface of the outer race is conformed to the inner race outer surface during formation of the outer race, and includes mating annular outer race axial thrust resisting areas mating with the annular inner race axial thrust resisting areas.
In U.S. Pat. No. 3,697,346, issued on Oct. 10, 1972, to H. B. Van Dorn et al, a method of making a composite thrust bearing is disclosed whereby a woven or braided fabric of a low friction material is applied over the outer surface of the inner bearing member. The body of the outer bearing member is built over the low friction surface by circumferentially wrapping resin-impregnated fiberglass over the fabric, curing and then finishing the outer member to desired axial and external conformations. The process integrally bonds the low friction fabric to the internal bearing surface of the outer member.
U.S. Pat. No. 4,054,337, issued on Oct. 18, 1977, to Matt et. al., and U.S. Pat. No. 4,040,883, issued on Aug. 9, 1977, also to Matt et. al., disclose thrust bearings comprising inner and outer races having a low friction fabric bonded to the inner surface of the outer race through a process comprising building up a filament wound body over the fabric, curing and then finishing to a desired external configuration.
In known methods for producing composite thrust bearings of filament wound outer races, the inner race outer surface grooves or the recessed areas between series of beaded elevations are filled by winding glass filaments at a low angle at or approaching circumferential or hoop windings. Unless such winding is used the filaments will bridge over the recessed areas which form the axial thrust resisting areas in the inner race outer surface. If bridging occurs, the annular axial thrust resisting areas on the inner race may be incompletely filled during the filament winding process, leaving voids which would reduce the ability to resist axial thrust forces. The presence of or the extent of such a defect could not easily be detected after only a few winding turns of the filament during the winding step in formation of the outer race.
A different problem arises, however, when the annular recessed areas are integrally filled by repeated low angled circumferential, or hoop windings of fiberglass filaments. When hoop windings are used the recessed areas will be filled with compacted filaments oriented in the same direction and lying in intimate contact with filaments above and below. Heating the fiberglass resin matrix cures the resin-fiberglass mix filling the annular recessed areas, fixing annular axial thrust resisting areas in the outer race which fill and mate with the annular inner race axial thrust resisting areas. The heat treatment of curing causes expansion of the inner race. During curing, the outer race of the fiberglass matrix is somewhat fluid and does not become solidified until the elevated cure temperature is achieved. At this elevated temperature the relative mating axial thrust resisting areas of the inner and outer races become fixed. Upon cooling, differential coefficients of expansion cause the steel inner race to contract more than the outer race of glass filaments. The coefficient of expansion for the composite fiberglass and resin material occupying the peripheral recessed areas in the inner race will approach that of the glass itself. After curing when the outer race is fixed into a rigid form, the cooling will cause very little relative contraction of the outer fiberglass occupying the peripheral recessed areas but will cause a relatively great contraction of the steel inner race. The coefficient of expansion of steel is approximately 6 to 6.3 times 10.sup.-6 inches/inch/.degree.F., while the coefficient of expansion for glass is on the order of 2 times 10.sup.-6 inches/inch/.degree.F.
Therefore, after cooling the annular outer race axial thrust resisting areas are tightly wedged within the mating annular inner race axial thrust resisting areas. This is because the coefficient of expansion of a steel inner race is approximately three times that of the cured fiberglass-resin composition of the outer race. Thus, the greater contraction of the steel member causes that portion of the outer member occupying the recessed areas of the peripheral surface of the inner member to be tightly compacted between annular thrust bearing surfaces. The wedge fit between the inner and outer members of the composite thrust bearing can become very tight, making it difficult or impossible to have relative rotational movement along the annular peripheral surface of the inner bearing member.